painting over oil based paint and drywall patches

I am re-doing one of the bedrooms in the home we recently purchased. Previous owner had some kind of overhang installed that i removed which left a number of holes that they had made for the wiring in it which i had to patch. To top it off for some reason, the room was painted with oil based paint.

What do I need to do to make sure this looks good?

I want to paint the finishing coat using latex but what primer do i use? I have sanded all the walls but how much sanding is required? Just a once over? because the walls still look fairly glossy.